Data communications between terminals in a mobile communication system

ABSTRACT

An apparatus and method are provided for communicating data between a first and a second terminal unit, wherein at least one of the terminal units is a mobile terminal unit. The apparatus includes a base station and a number of antenna units each linked to the base station. The base station has a transmitter that transmits modulated data signals to each of the antenna units, a receiver that receives modulated data signals forwarded by at least one of the antenna units, demodulator that demodulates received modulated data signals that have been modulated according to a predetermined modulation scheme. In the predetermined modulation scheme, successive blocks of modulated data are arranged such that a predetermined minimum time period elapses between the arrival, at the receiver, of a first and the arrival of a second of the successive modulated data blocks and wherein, in operation, the predetermined minimum time period being adjusted based on the maximum delay at the receiver between the arrival of a modulated data block a first antenna unit to a time of arrival of the same modulated data block of a different antenna unit.

FIELD

The present invention relates to data communications and in particularto a method and apparatus for communicating data between terminaldevices, at least one of which is a mobile terminal device.

BACKGROUND

In order to provide coverage over a particular area, mobilecommunications networks tend to be organised on a cellular basis, eachcell representing an area within which a mobile terminal device maycommunicate wirelessly with a corresponding cellular base station, eachcellular base station being interlinked by a communications network. Amobile terminal device moving from one cell, where it was communicatingvia a first base station, to another cell corresponding to a second basestation must undergo “handover” between the first base station and thesecond in order for the communication to continue once it moves out ofrange of the first base station. Each base station operates at adifferent frequency and hence the handover involves a change ofcommunication frequency. The process of handover can cause slightinterruptions to communication which, in the case of voice or othermobile telephony applications, is not a critical factor. However, ifapplied to higher data rate communications, for example to the streamingof live differentially-coded video, even slight interruptions incommunication of a few microseconds can result in irrecoverable dataloss and image degradation for some period of time beyond theinterruption.

SUMMARY

According to a first aspect of the present invention, there is providedan apparatus, operable to communicate data between a first and a secondterminal unit, wherein at least one of said first and second terminalunits is a mobile terminal unit, the apparatus comprising:

a base station; and

a plurality of antenna units having different areas of coverage, whereineach antenna unit is linked to said base station and is operable totransmit, wirelessly, modulated data signals received from said basestation, and to forward modulated data signals, received wirelessly, tosaid base station,

wherein said base station comprises:

a transmitter for transmitting modulated data signals to each of saidplurality of antenna units for wireless transmission;

a receiver for receiving modulated data signals forwarded by at leastone of said plurality of antenna units; and

a demodulator for demodulating received modulated data signals inrespect of a given data channel that have been modulated according to apredetermined modulation scheme, whereby, according to saidpredetermined scheme, successive blocks of modulated data in said datachannel are arranged such that a predetermined minimum time periodelapses between the arrival, at the receiver, of a first and the arrivalof a second of said successive modulated data blocks and wherein, for agiven arrangement of said plurality of antenna units, said predeterminedminimum time period is set to correspond to a time interval at saidreceiver from the time of first arrival to the time of latest arrival ofthe first of said successive modulated data blocks in said data channelby means of antenna units in said given arrangement.

Preferred embodiments of the present invention enable communicationswith a mobile terminal using only a single frequency, irrespective ofwhere the mobile terminal is located within the areas of radio coverageof the antenna units. Any potential problems arising through receptionof signals via different antenna units with correspondingly differentdelays are avoided by ensuring that delayed signals cannot interferewith each other during demodulation; allowances are made in themodulation scheme for the differing delays that would be expected. Thisenables a much simpler solution to such potential problems than thatemployed in conventional mobile communications systems where multiplecommunications frequencies are used.

In preferred embodiments of the present invention, coded orthogonalfrequency division multiplexing (COFDM) is used to modulate/demodulatesignals at the base station and in mobile terminals. COFDM modulationworks particularly well in environments with severe multipath signals bymaking use of so-called “guard band” delays. Any one of a number ofdifferent types of COFDM modulation may be used, of which DQPSK and64AQAM COFDM are particular examples. Preferably, forward errorcorrection is also used to help reduce multipath data errors.

According to a second aspect of the present invention there is provideda method of communicating data between a first, mobile, terminal unitand a second terminal unit over a data channel established by means of aplurality of antenna units, having different areas of coverage, and anassociated base station to the second terminal unit, the methodcomprising the steps of:

(i) at the first, mobile, terminal unit, generating a modulated datasignal according to a predetermined modulation scheme;

(ii) transmitting the modulated data signal wirelessly for reception byat least one of said plurality of antenna units; and

(iii) at the associated base station, demodulating the receivedmodulated data signal in said data channel for communication to thesecond terminal unit, wherein, at step (i), according to saidpredetermined modulation scheme, successive blocks of modulated data insaid data channel are arranged such that a predetermined minimum timeperiod elapses between the arrival, at the base station, of a first andthe arrival of a second of said successive modulated data blocks andwherein, for a given arrangement of said plurality of antenna units,said predetermined minimum time period is set to a time interval at thebase station from the time of first arrival to the time of latestarrival of the first of said successive modulated data blocks in saiddata channel by means of antenna units in said given arrangement.

Throughout the present patent specification, where the words “comprise”,“comprises” or “comprising”, or variations thereupon, are used they areto be interpreted to mean that the subject in question includes theelement or elements that follow, but that the subject is not limited toincluding only that element or those elements.

DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described inmore detail and by way of example only with reference to theaccompanying drawings, of which:

FIG. 1 shows in overview a fibre-radio communication apparatus accordingto preferred embodiments of the present invention;

FIG. 2 shows principal elements of a base station for use in preferredembodiments of the present invention;

FIG. 3 shows principal elements of a remote antenna unit for use inpreferred embodiments of the present invention;

FIG. 4 shows the components of a downlink transmitting interface of abase station according to a preferred embodiment of the presentinvention;

FIG. 5 shows the components of a downlink optical transmitter arrangedto transmit both local oscillator and data signals according to apreferred embodiment of the present invention;

FIG. 6 shows the components of a remote antenna unit according to apreferred embodiment of the present invention;

FIG. 7 shows the components of an uplink receiving interface accordingto a preferred embodiment of the present invention;

FIG. 8 shows the components of a mobile transmit/receive interfaceaccording to a preferred embodiment of the present invention;

FIG. 9 shows the components of a further design for the downlink opticaltransmitter according to a preferred embodiment of the presentinvention;

FIG. 10 shows a shaped-dielectric antenna suitable for use with a remoteantenna unit according to a preferred embodiment of the presentinvention; and

FIG. 11 shows a shaped-dielectric antenna suitable for use with a mobileterminal unit according to a preferred embodiment of the presentinvention.

DETAILED DESCRIPTION

Preferred embodiments of the present invention relate to an apparatusdesigned to provide a communications path between terminals, at leastone of which is a mobile terminal unit. In a preferred application, oneor more high bandwidth communications channels are to be provided toenable wireless communication between a central terminal and one or moremobile devices, for example high-definition television cameras movingwithin a relatively enclosed environment such as a large TV studio orfilm set. In such an environment, high-frequency signals, preferably ofthe order of 55-65 GHz, which when communicated wirelessly, are subjectto attenuation, distortion and other effects. Such effects are nottypically encountered, or not encountered to the same extent, inconventional mobile communications systems which operate with lowerfrequency signals and in more open environments. A preferred apparatuscomprises a base station and one or more remote antenna units (RAUs). Apreferred mobile terminal unit transmit/receive interface will also bedescribed for use with the preferred base station and remote antennaunits. An overview of the preferred apparatus and its operation will nowbe described with reference to FIG. 1.

Referring to FIG. 1, a base station 100 is arranged to communicate withone or more mobile data terminals 120, 125 by means of RAUs 110. EachRAU 110 is linked to the base station 100 by means of a downlink opticalfibre 115 and an uplink optical fibre 118 in a fibre-radio architecture.Optical fibre transmission is used for communication between the basestation 100 and RAUs 110, rather than an electrical transmission line(e.g. coaxial cable or electrical waveguide) or radio frequency (RF)transmission. This is particularly relevant at frequencies of the orderof 60 GHz, where electrical waveguide insertion loss is ˜1.5 dB/m andattenuation is approximately 12 dB/km in free space. The base station100 is arranged to modulate data signals received for example from acentral terminal unit 105 or other terminal device and to transmit themoptically, with low loss, to each of the RAUs 110 over the downlinkoptical fibres 115. Each of the RAUs 110 is arranged to convert thereceived optical signals into millimeter-wave signals for wirelesstransmission from their antennae. A target mobile data terminal 120, 125moving within the area of radio coverage 130 of one or more of the RAUs110 is then able to receive the transmitted signal.

In the uplink direction, a radio-frequency signal transmitted by amobile data terminal 120, 125 may be received by one or more RAUs 110.Each receiving RAU 110 is arranged to down-convert the received signalinto an intermediate frequency (IF) data signal and to opticallytransmit the IF data signal over the respective uplink optical fibre 118for reception by the base station 100. After demodulating the opticallycarried IF data signal the base station 100 outputs the resultantsignal.

Whereas, in preferred embodiments of the present invention, separatedownlink 115 and uplink 118 optical fibre transmission lines arespecified for simplicity, it is possible to combine downlink and uplinktransmission lines between the base station 100 and an RAU 110 in asingle optical fibre through use of appropriate multiplexing andmodulation techniques and interfaces to split and combine fibres at thebase station 100.

A number of RAUs 110 with overlapping radio coverage areas 130 arearranged to form a single-frequency cellular structure using a differentfrequency for each of the mobile data terminals 120, 125. This is incontrast to conventional cellular radio systems in which a differentfrequency would be allocated for use by each RAU 110 to communicate withmobile data terminals 120, 125 moving within its area of radio coverage130. Moreover, use of a single frequency per mobile in preferredembodiments of the present invention avoids the need for a controlsystem that would otherwise be needed, as in a conventional cellularradio system, to manage the handover of mobile data terminals 120, 125as they move from the radio coverage area 130 and hence thecommunication frequency of one RAU 110 to those of another. This helpsto ensure continuous communication with no interruption (essential forthe transmission of real-time high data rate digital video signals, forexample), often not possible with conventional multiple frequencycellular radio systems where brief interruptions are often experiencedas a mobile changes its frequency when it moves between cells.

Elements and operation of the base station 100 according to a preferredembodiment of the present invention will now be described in more detailwith reference to FIG. 2, and further with reference to FIG. 1.

Referring to FIG. 2, the base station 100 is seen to comprise two mainsections: a downlink transmitting interface 200 and an uplink receivinginterface 245. Optical outputs from the downlink interface 200 andoptical inputs to the uplink interface 245 are joined by means of anappropriate interface to the optical fibres 115 and 118 respectivelylinking each of the RAUs 110 to the base station 100. Data signalsintended for a particular target mobile data terminal 120, 125 arereceived by the downlink transmitting interface 200 of the base station100 where a number of modulators 205 are provided, each one dedicated tomodulating input data signals in respect of a different data channel. Adata channel may be used to communicate with one or more mobile terminalunits 120, 125 according to the bandwidth requirements of thoseterminals. However, in a preferred embodiment of the present inventiondirected to a TV or film studio application, it is likely that a singlemobile terminal unit 120, 125 would require the entire bandwidth of adata channel for its own use, at least in an uplink direction. The basestation 100 would be equipped to provide as many data channels asrequired by the particular application. However, limitations infrequency availability would ultimately limit the number of channelsthat may be provided. In preferred embodiments of the present invention,use of the 55-65 GHz band provides sufficient bandwidth to handle anumber of high data rate duplex channels.

After modulation by an appropriate modulator 205 the modulated inputsignal is input to a downlink signal converter 210 where modulatedsignals for the respective data channel are converted to a predeterminedfrequency allocated specifically for the channel. The converted signalis then input to an optical transmitter and local oscillator 215arranged to generate a downlink optical signal, preferably comprising anoptical oscillator signal that is modulated by the converted inputsignal for transmission to the RAUs 110. Preferably, the downlinkoptical signal output by the optical transmitter 215 includes a separatelocal oscillator signal that is then available for use, after isolation,by each receiving RAU 110, so avoiding the need to deploy an oscillatorof the same frequency at each RAU 110. This reduces complex and bulkycircuitry for generating and controlling a local oscillator signalwithin each RAU 110. This proves advantageous as the RAUs 110 arepreferably designed to be small and compact so that they may be placedfor example in environments, e.g. lamp posts in certain applications,where the temperature may vary significantly and may make an LO signalunstable. The downlink optical signal is input to an optical splitter220 where it is divided and injected into each of the downlink opticalfibre links 115 by means of an appropriate interface to be conveyed toeach of the RAUs 110.

Where the number of RAUs 110 is such that use of a single opticalsplitter 220 is either impractical or results in excessively weakdownlink optical signals being injected into each of the downlink fibres115, considering the length of fibre 115 being used, an alternativetechnique for dividing the downlink optical signal may be implemented inwhich lower-order splitters, e.g. 1:4, are deployed in a cascadedarrangement, with erbium-doped fibre amplifiers being used to boost thesignal if required. For example, an initial splitter 220 at the basestation 100 may be linked to remote splitters located nearer to theparticular RAUs 110 being served to further sub-divide the signals.

In the uplink direction, any signals received by one or more RAUs 110from a mobile data terminal 120, 125 are converted and forwarded to thebase station 100 over the uplink optical fibres 118 to arrive at theuplink receiving interface 245. The uplink receiving interface 245includes a set of photo-receivers 225, one photo-receiver for eachuplink optical fibre 118, which detects and converts uplink opticalsignals arriving over the uplink optical fibres 118 into IF signals forinput to a channel separator 230. Uplink optical signals may comprise acombination of signals for one or more data channels which need to beseparated by the base station 100. The channel separator 230 istherefore designed to separate the signals for each data channel (andhence for the different mobile data terminals 120, 125) on the basisthat the signal for each data channel has a different predeterminedfrequency. Separated signals for each channel are then input to uplinksignal converters 235 where the signals at their respectivepredetermined frequencies are converted for input to demodulators 240, adifferent demodulator 240 for each data channel. The demodulated outputof each demodulator 235 forms the output from the base station 100, forexample to the central terminal unit 105.

Operation of the RAUs 110 will now be described in a little more detailwith reference to FIG. 3, and further with reference to FIG. 2.

Referring to FIG. 3, an RAU 110 is provided with a downlink opticalreceiver 310 and an uplink optical transmitter 335, each linked by meansof an optical interface 305 to the downlink optical fibre 115 and uplinkoptical fibre 118 respectively that connect the RAU 110 to the basestation 100. The downlink optical receiver 310 is arranged to receivedownlink optical signals transmitted by the base station opticaltransmitter and local oscillator 215 and to convert the received opticalsignals into radio frequency (RF) signals. The RF signals are input to adiplexer 312 arranged to separate the local oscillator signal generatedby the base station optical transmitter 215 from the data signals forone or more data channels. The data signals output by the diplexer 312are amplified by an amplifier 315 and fed to an antenna 320 for wirelesstransmission by the RAU 110.

In the uplink direction, any RF signal transmitted by a mobile dataterminal 120, 125 and received at an antenna 325 is passed to an uplinksignal converter 330 arranged to convert the received RF signal into anintermediate frequency (IF) data signal. The uplink signal converter 330uses the local oscillator signal separated by the diplexer 312 toconvert the received RF signal into the IF data signal which in turn ispassed to the uplink optical transmitter 335 to generate an uplinkoptical signal for transmission to the base station 100 over the uplinkoptical fibre 118. Preferably the uplink optical transmitter 335transmits the IF data signal either by directly modulating a laser diodeor by modulating the light from a (CW) laser diode in an externaloptical modulator. In particular applications it may be more convenientto use wavelength division multiplexing at the RAU 110 and wavelengthdivision demultiplexing at the base station 100 so that multiple uplinkoptical signals may be combined onto a single uplink optical fibre 118serving all the RAUs 110, or at least onto a reduced number of uplinkoptical fibres 118. However, in that case, the laser diode used in theuplink optical transmitter 335 would need to be selected so as to emitlight of a wavelength compatible with the wavelength divisionmultiplexer and with the associated channel spacing.

Whereas FIG. 3 shows a different antenna (320) being used at an RAU 110for transmitting signals to that (325) used for receiving signals, thesame physical antenna may be used for both transmitting and receiving.

As mentioned above, a different predetermined frequency is allocated toeach data channel provided by the base station 100 and RAUs 110. The useof a different frequency per data channel provides one of the preferredelements in embodiments of the present invention that enables a singlefrequency (per mobile data terminal 120, 125) mobile communicationsnetwork to be operated. Another preferred element enabling the singlefrequency network to operate is the choice of modulation techniqueimplemented by the modulators 205 and demodulators 240 in the basestation 100 and replicated in each of the mobile data terminals 120,125.

In a single frequency communications arrangement based upon thearchitecture shown in FIG. 1 in which the areas of radio coverage 130 ofthe RAUs 110 may overlap, a transmitted signal may be received by amobile data terminal 120, 125 from two or more different RAUs 110delayed by slightly different amounts due to their differing distancesfrom the mobile data terminal 120, 125. For example, referring to FIG.1, it can be seen that while the mobile terminal unit 120 lies withinthe radio coverage area 130 of a single RAU 110—“RAU 4”—the other mobileterminal unit 125 lies within a region of overlapping radio coverage fortwo RAUs 110—“RAU 2” and “RAU 3”. Similarly, a signal transmitted by amobile data terminal 120, 125 may be received by more than one RAU 110located within range of the mobile terminal so that each received signalwould be forwarded to arrive at the base station 100 at slightlydifferent times. In each case, the modulation scheme chosen should beinherently tolerant of such signal delays so that received signals maybe combined and successfully demodulated by the mobile data terminal120, 125 in the downlink direction and, in the uplink direction, by thebase station 100.

In preferred embodiments of the present invention, the modulation schemeselected is the Coded Orthogonal Frequency Division Multiplexing (COFDM)scheme as described, for example, in a book by Mark Massel, entitled“Digital Television: DVB-T COFDM and ATSC 8-VsB”, published byDigitaltvbooks.Com, ISBN 0970493207. One of the key features of COFDMthat enables modulated data signals to be received with differingdelays, combined and successfully demodulated, is the use of so-calledguard intervals in the modulated data signals.

COFDM is a form of multi-carrier digital modulation wherein data aremodulated onto a large number of closely-spaced carriers whoseseparation in the frequency domain is carefully chosen so that eachcarrier is orthogonal to the other carriers, so eliminating interferencebetween them when transmitted simultaneously. Each carrier is arrangedto send one symbol at a time. The time taken to transmit a symbol iscalled the symbol duration. In order to ensure that there is nointer-symbol interference on a particular carrier due to the delayedarrival at a receiver of a first symbol from two or more differentantennae, the symbol duration may be extended by the modulator by theinsertion of a so-called guard interval of predetermined length betweentransmitted symbols on the particular carrier to ensure that the nextsymbol on the carrier arrives at the receiver after the last delayedarrival of the first symbol.

Preferably, each of the downlink optical fibres 115 and each of theuplink optical fibres 118 are of substantially equal length so as tominimise differential time delays in conveying signals between the basestation 100 and each of the RAUs 110.

The downlink transmitting interface 200 of the base station 100 will nowbe described in more detail, according to a preferred embodiment of thepresent invention, with reference to FIG. 4. The same reference numeralsare used to label features shown in FIG. 4 that are similar to those inany of the earlier figures. In this preferred embodiment, the basestation 100 provides two communications channels. This two-channelexample will be used as the basis for the remainder of the descriptionin the present patent application in order to simplify the figures,although, of course, the base station 100 may be equipped to providefurther data channels as required, as will become clear from thedescription that follows.

Referring to FIG. 4, components of a preferred two channel downlinktransmitting interface 200 are shown. In particular, two modems(modulators) 205 are provided, one for each data channel. To communicatewith a particular one of the mobile data terminals 120, 125, anappropriate one of the two data channels is selected and data is inputto the respective modem 205 for that channel. The modem 205 modulatesthe input data signal, preferably according to the COFDM modulationscheme. Though not shown in FIG. 4, preferably the “I” and “Q” channeloutputs from a (COFDM) modem 205 are converted into a combined firstintermediate frequency channel by mixing each of the “I” and “Q” signalswith a 520 MHz intermediate frequency (IF) oscillator signal, the “Q”signal being mixed with a 520 MHz IF oscillator signal that is a quartercycle out of phase with that for the “I” signal, and combining theresultant signals. The combined signal from each modem 205 is passedthrough a 520 MHz band-pass filter 405 having a bandwidth ofapproximately 340 MHz, to remove any unwanted harmonics and noise thatwould typically be generated as a result of the preferred IF mixing andcombining stage.

The signal output from the filter 405 for each channel is then input tothe downlink signal converter 210 for conversion into a signal of apredetermined frequency allocated for that data channel, preferably inthe range 1.5 to 3.5 GHz. The downlink signal converter 210 comprises,for each data channel, a mixer 410 and a local oscillator (LO) 415, 418.The frequencies of the local oscillators 415, 418 are selected to ensurethat when the oscillator signal is mixed (410) with the output signalfrom the filter 405, a signal of the predetermined frequency for thatchannel is generated. Preferably, the frequencies of the localoscillators 415, 418, and hence the predetermined frequencies for thechannels, are selected so as to minimise unwanted mixing productsgenerated as a result of mixing the signal from the local oscillators415, 418 with the output signals from the filters 405, bearing in mindthe particular combination of frequencies used to generate those outputsignals. In the present example, having two data channels, the localoscillator 415 for one of the channels is preferably set to a frequencyof 1.43 GHz and the local oscillator 418 for the other channel is set toa frequency of 2.68 GHz. If the base station 100 were to be equipped toprovide n data channels, then n modems 205, filters 405, mixers 410 andlocal oscillators 415, 418, would typically need to be provided, eachlocal oscillator being set to a different frequency such as to generatea channel signal within a predetermined frequency range, e.g. 1.5-3.5GHz. The process of selecting channel frequencies and hencecorresponding oscillator frequencies takes place as part of an overalldesign stage for the apparatus. However, while the use of fixed localoscillator frequencies is discussed in the present example, a switchingarrangement can be implemented to enable different local oscillators tobe selected to enable switching between data channels and hencecommunication with different mobile terminal units 120, 125.Alternatively, tunable local oscillators may be provided to achieve asimilar effect.

The output from the mixer 410 comprises not only a signal at theallocated frequency for the data channel but also signals at one or moreother frequencies. A filter 420, 423 is used therefore to remove theunwanted components from the mixer output signal leaving only a signalof the allocated frequency for the data channel. In the present example,the filters 420 and 423 are band-pass filters centred on frequencies of1.95 GHz and 3.2 GHz respectively, both having a bandwidth greater thanor equal to 340 MHz. The signals emerging from the filters 420 and 423,each of a distinct frequency, are combined in a combiner 425 to form acomposite signal for input to the optical transmitter 215. The combiner425 in the present example is a 2:1 combiner because there are only twodata channels. If the base station 100 was equipped to provide nchannels, then an n:1 combiner would be provided to combine the signalsinto a single composite channel.

In a preferred embodiment of the present invention, the opticaltransmitter 215 is constructed according to a cascaded optical modulatordesign. An optical carrier generated by a laser 430 is optically coupledusing polarisation maintaining optical fibre to a first opticalmodulator 440 arranged to modulate the optical carrier with an amplified(437) and filtered (439) oscillator signal generated by an oscillator435 to form an optical oscillator signal and, in a second opticalmodulator 445, optically coupled using polarisation maintaining opticalfibre to the first optical modulator 440, the optical oscillator signalis modulated with an amplified (447) and filtered (449) composite signaloutput by the combiner 425. The frequency of the oscillator 435 isselected to ensure that a signal is output from the second opticalmodulator 445 having a predetermined frequency suitable for wirelesstransmission by the RAUs 110. This predetermined frequency would berequired to fall within a range of frequencies for which a license totransmit has been granted. In preferred embodiments of the presentinvention this range of frequencies is chosen to be 57-59 GHz for thedownlink and 62-64 GHz for the uplink, with a local oscillator frequencyof 60.5 GHz. The downlink optical signal output by the second opticalmodulator 445 is split by the optical splitter 220 and injected intoeach of the downlink optical fibres 115 linking the base station 100with the RAUs 110.

Operation of the optical transmitter 215 will now be described in moredetail according to a preferred embodiment of the present invention withreference to FIG. 5.

Referring to FIG. 5, the optical modulators 440 and 445 are preferablycommercially available high frequency Mach-Zehnder (MZ) opticalmodulators. The first optical modulator 440 is biased at the minimum ofits transfer characteristic so that a frequency-doubling effect can beachieved in modulating the laser light (430), preferably output by 50 mWDFB laser diode 430, with the amplified oscillator signal (435, 437,439). Frequency doubling may be achieved by biasing the first opticalmodulator 440 at either its maximum or minimum. However, it ispreferable to bias at the minimum point as this minimises the dc lightlevel at a photo-receiver and thus provides the best noise performance.Making use of the frequency doubling properties of a MZ modulatorenables an oscillator 435 having a frequency of only 30.25 GHz to beused to generate a 60.5 GHz oscillator signal in the optical output fromthe first MZ optical modulator 440—in fact two optical oscillatorsideband signals are generated, as shown (505) in FIG. 5, separated by60.5 GHz—the laser carrier itself (430) being suppressed. The second MZoptical modulator 445 is biased at the quadrature point, the most linearregion of its transfer characteristic. When the amplified composite IFdata signal is input to the second MZ optical modulator 445 each of theoptical oscillator sidebands is modulated resulting in a pair of opticaldata signal sidebands centred about each of the optical oscillatorsidebands, as shown (510) in FIG. 5, the first pair in the frequencyrange 57-59 GHz and the second in the range 62-64 GHz respectively inthe present example, corresponding to the composite IF data signalfrequency range of 1.5 to 3.5 GHz. Each data signal sideband isseparated, in the frequency domain, from the optical oscillator sidebandsignals according to the frequencies of the signal components within thecomposite IF data signal. The downlink optical signal output by thesecond MZ optical modulator 445 is then injected into each of thedownlink optical fibres 115 for sending to the RAUs 110.

Operation of an RAU 110 will now be described in more detail, accordingto a preferred embodiment of the present invention, with reference toFIG. 6.

Referring to FIG. 6, the downlink optical signal output by the opticaltransmitter 215 at the base station 100 is received over the downlinkoptical fibre 115 at an optical interface 305 and passed to an opticalreceiver 310 comprising a photo-receiver 605. The RF electrical outputsfrom the photo-receiver 605 are the 60.5 GHz local oscillator signal, asgenerated by the base station optical transmitter 215, and the lower andupper data signal sidebands in the frequency ranges 57-59 GHz and 62-64GHz respectively (60.5 GHz±1.5-3.5 GHz). The RF signals are amplified inan amplifier 610 and input to a diplexer 312 arranged to separate thelocal oscillator signal from the data signal sidebands. Preferably, inthe present example, the lower frequency sideband in the range 57-59 GHzis retained as the downlink signal for transmission by the RAU 110,while the upper frequency sideband is blocked by means of a band-passfilter 615 that permits only the lower frequency band to pass to thepower amplifier 315 and then by means of an isolator 620 to the downlinkantenna 320. The separated local oscillator signal is passed to theuplink signal converter 330 for use in converting received mm-waveuplink signals into IF uplink signals.

In the uplink direction a mm-wave signal transmitted by a mobile dataterminal 120, 125 and received at the RAU 110 by the antenna 325 ispassed by means of an isolator 635 to the uplink signal converter 330.The received uplink signal is first filtered in a band-pass filter 640arranged to allow signals in the range 62-64 GHz to pass—the preferredfrequency range for uplink communications in the present example—thenamplified in an amplifier 645 and input to a mixer 650. The separated60.5 GHz local oscillator signal from the diplexer 312 is filtered in a60.5 GHz band-pass filter 625 and amplified by an amplifier 630 beforeinput to the mixer 650. The result of mixing the 60.5 GHz localoscillator signal with the received uplink signal is, amongst othermixing products, an uplink IF signal in the frequency range 1.5-3.5 GHz.The mixer output is filtered in the band-pass filter 625 and amplifiedin an amplifier 655 before filtering out all but the uplink IF signal inthe frequency range 1.5-3.5 GHz in a band-pass filter 660. After furtheramplification in an amplifier 665 the uplink signal converter 330outputs the uplink IF signal to the uplink optical transmitter 335. Theuplink optical transmitter 335 comprises an optical modulator 670 tomodulate the uplink IF signal onto an optical carrier signal provided bya laser 675 to generate an uplink optical signal which is then injectedinto the uplink optical fibre 118 to the base station 100.

The uplink receiving interface 245 of the base station 100 will now bedescribed in more detail according to a preferred embodiment of thepresent invention with reference to FIG. 7. The same reference numeralsare used to label features shown in FIG. 7 that are similar to those inany of the earlier figures.

Referring to FIG. 7, components of a preferred two channel uplinkreceiving interface 245 are shown. The uplink receiving interface 245 inthis example is arranged to interface with any combination of three RAUs110, although of course the base station 100 may be scaled to interfacewith further RAUs 110 as will be clear from this description. Uplinkoptical signals received over any of the three uplink optical fibres 118are detected by a photo-receiver 225 linked to that uplink optical fibre118 by an appropriate interface. The photo-receiver 225 converts thereceived uplink optical signal into an uplink IF signal similar to thatgenerated by the uplink signal converter 330 within the RAU 110. Adifferent photo-receiver 225 is provided to receive signals from each ofthe three uplink optical fibres 118. The uplink IF signal output by eachof the photo-receivers 225 is then input to the channel separator 230.Uplink optical signals received from an RAU 110 may carry signals formore than one data channel simultaneously if the RAU 110 was withinrange of multiple transmitting mobile terminal units 120, 125. Thechannel separator 230 is designed to separate the signals for each ofthe data channels and, where signals for a given data channel areseparately received from more than one RAU 110, to combine all thereceived signals for a given data channel so as to output a combinedchannel signal for each channel. Thus, in the example shown in FIG. 7,the three uplink optical fibre inputs 118 convert to two channel outputsfrom the channel separator 230.

The signals for each data channel are distinguished by their differingfrequencies. Hence, after amplification in an IF amplifier 705, thechannel separator 230 splits the uplink IF signal from eachphoto-receiver 225 along two signal paths, one signal path per datachannel, using a splitter 710. In the present example, one signal pathleads to a 1.95 GHz band-pass filter 715 to pass signals at theallocated frequency for the first data channel and the other signal pathleads to a 3.2 GHz band-pass filter 720 to pass signals at the allocatedfrequency for the second data channel. Signals passed by each of thethree first band-pass filters 715 shown in FIG. 7 for the first datachannel are combined in a 3:1 combiner 725 (if there were n RAUs 110,then the combiner 725 would be an n:1 combiner) and similarly for thethree band pass filters 720 for the second data channel in a different3:1 combiner 728. The combined uplink IF signals for each data channelare each then amplified in IF amplifiers 730 and 732, filtered again infurther respective band-pass filters 735, 738, similar to filters 715and 720 respectively, to remove any signal components generated by thecombiner 725 at frequencies other than the desired channel frequencies.After filtering, the combined signals for each data channel are output,separately, to the uplink signal converter 235.

The uplink signal converter 235 comprises, for each data channel, amixer 740, 742 and a local oscillator 745, 748. The local oscillators745, 748 operate at the same frequencies as the local oscillators 415and 418 respectively in the downlink transmitting interface 210described above. The combined uplink IF signals for each channel arereceived at the respective mixer 740, 742 and mixed with thecorresponding local oscillator signals. The resultant signals are thenamplified by a respective IF amplifier 750, 752. The mixers 740, 742generate a number of signal components of which only one is required.Therefore a band-pass filter 755, 758 is used to block the unwantedsignal components for each channel before the required uplink signalcomponents are output to be demodulated in respective COFDM demodulators240.

Preferably, the modems 240 are COFDM modems. The demodulated data signalfor each channel is then output from the modem 240, for example to thecentral terminal unit 105.

A preferred mobile transmit/receive interface will now be described,with reference to FIG. 8, for use in a mobile terminal unit 120, 125 toenable communication with the base station 100 via the RAUs 110. In apreferred application, the mobile transmit/receive interface may bephysically mounted and electronically connected to a movable televisioncamera to enable the camera to transmit image data to and receivecontrol data from a central studio, for example, by means of the RAUs110 and base station 100.

Referring to FIG. 8, components in a preferred mobile terminal unit 120,125 are shown, including a data source 805, a TV camera for example,linked for uplink communications to the mobile transmit/receiveinterface 810 by means of a COFDM modulator 815. A downlink signaloutput from the mobile transmit/receive interface 810 is demodulated ina COFDM demodulator 820 for output (825) to a TV monitor, for example.Both the COFDM modulator 815 and demodulator 820 are arranged tocooperate with the demodulators 240 and modulators 205 respectively, asused in the base station 100. Although not shown in FIG. 8, the COFDMmodulator 815 includes circuitry to convert a baseband modulated signalinto an IF uplink data signal of a predetermined frequency specific tothat mobile transmit/receive interface 810, either 1.95 GHz or 3.2 GHzin the present two-channel example. Similarly, the COFDM demodulator 820includes circuitry to convert a downlink IF data signal into a signal ofthe required frequency for demodulation by the COFDM demodulator 820.This assumes of course that the mobile transmit/receive interface isgoing to be used to communicate on only one of the data channelssupported by the base station 100, although a switching arrangement canbe provided at the mobile terminal unit 120, 125 if required to enableswitching between channel frequencies in a similar manner to thatmentioned above in describing the operation of a preferred base station100.

Considering the uplink direction first, a signal input by the datasource 805 is COFDM modulated and converted (815) into an IF uplink datasignal. The mobile transmit/receive interface 810 receives the uplink IFdata signal and amplifies it in an IF amplifier 830 and mixes theamplified signal in a mixer 835 with a 60.5 GHz local oscillator signal,in the present example, generated by a local oscillator 840. The mixeroutput is then filtered in a band-pass filter to block all but thosemixer products in the preferred uplink wireless communication frequencyrange of 62-64 GHz. After amplification in a power amplifier 850, theuplink data signal is transmitted wirelessly by means of an antenna 855to be received by one or more RAUs 110.

In the downlink direction, a signal transmitted by one or more RAUs 110,in the preferred downlink wireless communication frequency range of57-59 GHz for the present example, is received at an antenna 860. Thereceived downlink signal is filtered in a 57-59 GHz band-pass filter 865and amplified in a low-noise amplifier (LNA) 870 before input to a mixer875 arranged to mix the amplified signal with the local oscillatorsignal from oscillator 840. One of the results of mixing the oscillatorsignal with a signal in the range 57-59 GHz is a downlink IF data signalin the frequency range 1.5-3.5 GHz. All other mixer products are blockedin a band-pass filter 880, leaving the downlink IF data signal to beamplified in an IF amplifier 885 for output from the mobiletransmit/receive interface. 810. The output IF data signal is convertedand demodulated in the COFDM demodulator 820 and output (825), forexample to a TV monitor.

An alternative design for the downlink optical transmitter and localoscillator 215 will now be described, according to a preferredembodiment of the present invention, with reference to FIG. 9. Thosecomponents shared in common with the transmitter 215 described abovewith reference to FIG. 4 and FIG. 5 are labelled with the same referencenumerals.

Referring to FIG. 9, a preferred optical transmitter is shownconstructed according to a so-called RF single sidebandfrequency-doubled design. In this design the composite signal output bythe combiner 425 is firstly filtered in a 1.5-3.5 GHz band-pass filter449 before input to a single-sideband non-suppressed carrier electricalmodulator 905 to modulate an RF oscillator signal generated by theoscillator 435. It is important that the oscillator carrier signal isnot suppressed by the modulator as the oscillator signal will beincluded in the signal transmitted to the RAUs 110. The resultingsingle-sideband signal and the oscillator signal output by the modulator905 are further amplified (910) and filtered in a 30.5 GHz low-passfilter 915 to provide additional rejection of any unwanted uppersideband signal. The resultant single-sideband signal and oscillatorsignal, shown (920) in FIG. 9, are input to a MZ optical modulator 440biased at the minimum of its transfer characteristic, as for the firstoptical modulator in the cascaded optical modulator design describedabove with reference to FIG. 5, so as to achieve frequency doubling andsuppression of the optical carrier input from a laser 430. The laser 430is optically coupled using polarisation maintaining optical fibre to theMZ optical modulator 440 where the optical carrier is modulated by thesingle-sideband and oscillator signal (920). The result (shown as 925 inFIG. 9) is a downlink optical signal comprising a pair of localoscillator signals separated by 60.5 GHz together with two downlink datasidebands separated, in the frequency domain, from the oscillator signalaccording to the frequency of the single-sideband signal (920). Althoughthe frequencies of the single-sideband and oscillator signals input tothe MZ modulator 440 are doubled, the frequency separation of theoscillator and sideband signal components is maintained aftermodulation—an important feature that enables this design of opticaltransmitter 215 to be used as an alternative to the cascaded opticaltransmitter design described above with reference to FIG. 5 withoutneeding to modify the design of the other components of the apparatus orthe mobile terminal units 120, 125. The downlink optical signal outputby the MZ optical modulator 440 is shown as 925 in FIG. 5. This signalis injected into the downlink optical fibres 115 for communication tothe RAUs 110.

A preferred application of the apparatus described above according topreferred embodiments of the present invention will now be described inoutline. This preferred application was alluded to above and concernsthe wireless communication of signals from television or film cameras ina TV studio or film set environment. In such an environment,particularly one comprising a number of distinct studios, signalstransmitted wirelessly by RAUs 110 at a frequency of approximately 60GHz, as discussed throughout the example presented in the descriptionabove, would be essentially constrained to particular studios. Even infree space, such signals are subject to attenuation at the rate of 12dB/km. Thus, the possibility of multipath signals can be significantlyreduced, particularly where shaped radiation pattern antennae are usedin both the RAUs 110 and the mobile transmit/receive interfaces 810 toreduce reflections from studio walls, etc.

Preferred designs for shaped radiation pattern antennae will now bedescribed according to preferred embodiments of the present invention.Firstly, a preferred design for use as an antenna unit 320, 325 for anRAU 110 will be described with reference to FIG. 10 and secondly apreferred design for use as an antenna 855, 860 for a mobile terminalunit 120, 125 will be described with reference to FIG. 11. Preferably,each of the antennae are designed for use with signals in the frequencyrange 57 to 64 GHz, although it would be apparent to a person ofordinary skill in the field of antenna design that the antennae may bedesigned to operate in other frequency ranges according to theparticular application of the apparatus of the present invention.

Referring to FIG. 10 a, a plan view of a preferred shaped radiationpattern antenna 1000 is shown. The preferred antenna 1000 is arotationally symmetric shaped-dielectric lens antenna comprising adielectric lens portion 1005, preferably made from PTFE, mounted on aconducting mounting plate 1010. The dielectric lens 1005 is of a knownshape designed to produce a substantially sec²θ radiation power pattern,where θ is the angle measured from the axis of symmetry through theantenna 1000, for angles of θ up to approximately 70°. This powerpattern has been found to be suitable for use in an enclosed environmentsuch as a television studio where the antenna is attached to the ceilingnear to the centre of the space. This design forms a good compromise foruse in such environments over an alternative known, but more complex,lens design capable of producing substantially rectangular radiationfields.

Referring to FIG. 10 b, a plane section through the antenna 1000 isshown, taken through the plane indicated by the line A-A in FIG. 10 a.The shaped dielectric lens 1005 is attached to the conducting mountingplate 1010 by means of four fixing bolts 1015, each made, optionally,from a similar material to that used for the dielectric lens 1005itself, although metal bolts may also be used. Each bolt 1015 engageswith a corresponding threaded hole provided in a projecting annularportion 1016 of the dielectric lens 1005 which itself engages with acorresponding annular recess 1018 provided in the mounting plate 1010. Ahole 1020 is provided through the centre of the mounting plate 1010 toprovide a point of entry for a waveguide 1025 assembly. The waveguideassembly 1025 comprises an air-filled polariser, of conventional design,arranged in two parts to emit radiation with circular polarisation intothe dielectric lens: a rectangular-sectioned portion 1030 leading to aflattened circular sectioned portion 1035, with appropriately shapedtransition sections 1040 and 1045 disposed between the rectangular 1030and flattened circular 1035 air-filled sections and between theair-filled flattened circular 1035 and dielectric-filled entry hole1020, respectively. That portion of the hole 1020 not occupied by thewaveguide feeder transition section 1045 is filled with dielectricmaterial, preferably the same material as that used for the lens 1005itself. Preferably, a portion of the dielectric material may have acentral bore or alternatively have its external radius reduced in orderto provide an impedance matching section between the air-filled circularwaveguide and dielectric-filled entry hole.

Preferably, an axially-symmetric pattern of circular grooves 1050 is cutinto the surface of the dielectric lens to help to reduce the effects ofinternal reflections within the lens, in a known manner.

Referring to FIG. 11 a, a plan view of a preferred shaped radiationpattern antenna 1100 is shown for use with a mobile terminal unit 120,125. The preferred antenna 1100 is a rotationally symmetricshaped-dielectric lens antenna comprising a dielectric lens portion1105, also preferably made from PTFE, mounted on a conducting mountingplate 1110. The dielectric lens 1105 is shaped according to a knownshape designed to produce a substantially hemispherical radiation powerpattern.

Referring to FIG. 11 b, a plane section through the antenna 1100 isshown, taken through the plane indicated by the line B-B in FIG. 11 a.The shaped dielectric lens 1105 is attached to the conducting mountingplate 1110 by means of a projecting annular portion 1115 which engageswith a corresponding annular recess 1118 provided in the mounting plate1110. A hole 1120 is provided through the centre of the mounting plate1110 as a point of entry for a waveguide 1125 assembly. The waveguideassembly 1125 is similar in design to that (1025) used with the RAUantenna 1000 of FIG. 10, although with a smaller diameter feed 1130 intothe dielectric lens 1105 to give a wider radiation pattern and hence awider illumination of the lens 1105. However, in providing a widerillumination within the lens 1105 the effect of internal reflections onthe radiation pattern has been found to be greater than that with theRAU antenna 1000, in particular on the radiation pattern towards theouter limits of the field between 70° and 90° as measured from the axisof symmetry of the lens. It is has been found, however, that if anannular portion 1135 of a radiation absorbing material, for exampleEmerson & Cuming “Eccosorb AN-72”™, is disposed in an annular recessformed towards the outer edge of the mounting plate 1110, a recessformed preferably by extending the width of the recess 1118 radiallyoutwards, then the effect of the internal reflections can beconsiderably reduced. Preferably, the projecting annular portion 1115 ofdielectric material together with the annular portion of absorbermaterial 1135 together fill the extended annular recess 1118 in themounting plate 1110 to provide a secure attachment of the dielectriclens 1105 to the mount 1110.

As with the RAU antenna 1000, the surface of the dielectric lens 1105 ofthe mobile terminal unit antenna 1100 is provided with a pattern ofcircular grooves 1140 to reduce internal reflections.

Whereas, in some applications, a single mobile terminal unit 120, 125may require the entire bandwidth of a data channel, in otherapplications a number of mobile terminal units may share a given datachannel and the associated base station equipment by using a combinationof Time Division Multiplexing (TDM) and Frequency Division Multiplexing(FDM). This would involve allocating time intervals to a group of mobileusers who all operate at one frequency. There would be a number of these‘groups’ operating at different frequencies. However, whereas aconventional cellular radio system is designed to support low bandwidthcommunication by millions of mobile users, the apparatus according topreferred embodiments of the present invention is intended for usernumbers of the order of hundreds.

1. An apparatus, operable to communicate data between a first and asecond terminal unit, wherein at least one of said first and secondterminal units is a mobile terminal unit, the apparatus comprising: abase station; and a plurality of antenna units having different areas ofcoverage, wherein each antenna unit is linked to said base station andis operable to transmit, wirelessly, modulated data signals receivedfrom said base station, and to forward modulated data signals, receivedwirelessly, to said base station, wherein said base station comprises: atransmitter for transmitting modulated data signals to each of saidplurality of antenna units for wireless transmission using a singlefrequency band of a given data channel; a receiver for receivingmodulated data signals forwarded by at least one of said plurality ofantenna units at the single frequency band of said given data channel;and a demodulator for demodulating received modulated data signals inrespect of said given data channel that have been modulated according toa predetermined modulation scheme, whereby, according to saidpredetermined scheme, successive blocks of modulated data in said datachannel are arranged such that a predetermined minimum time periodelapses between the arrival, at the receiver, of a first and the arrivalof a second of said successive modulated data blocks and wherein, for agiven arrangement of said plurality of antenna units, said predeterminedminimum time period is set to correspond to a time interval at saidreceiver from the time of first arrival to the time of latest arrival ofthe first of said successive modulated data blocks in said data channelby means of antenna units in said given arrangement such that all datain a given data channel is communicated via at least one of theplurality of antenna units at one time.
 2. An apparatus according toclaim 1, wherein the time of latest arrival of the first of saidsuccessive modulated data blocks in said data channel is the time oflatest arrival of the first of said successive modulated data blocks ata signal level sufficiently high to cause interference with the secondof said successive modulated data blocks arriving in said data channel.3. An apparatus according to claim 1, wherein each of said plurality ofantenna units is linked to said base station by means of at least oneoptical fibre transmission line and wherein said transmitter and saidreceiver are electro-optical devices.
 4. An apparatus according to claim3, wherein each of said plurality of antenna units is linked to saidbase station by means of a substantially equal length of optical fibretransmission line so as to equalise the delays in communicating signalsbetween said base station and each of said plurality of antenna units.5. An apparatus according to claim 1, operable to provide at least twodata channels for communicating with said at least one mobile terminalunit, wherein a different predetermined frequency is allocated forcommunicating data over each of said data channels.
 6. An apparatusaccording to claim 5, wherein each of said predetermined frequencies liein the frequency range 55 GHz to 65 GHz.
 7. An apparatus according toclaim 6, wherein said base station further comprises channel separatingmeans for separating signals relating to each of said at least two datachannels when contained within a modulated data signal received at thereceiver from one of said plurality of antenna units.
 8. An apparatusaccording to claim 7, wherein said base station further comprisescombining means operable, for each of said at least two data channels,to combine respective separated signals received from at least two ofsaid plurality of antenna units.
 9. An apparatus according to claim 1,wherein said predetermined modulation scheme is a coded orthogonalfrequency division multiplexing (COFDM) scheme and wherein saidpredetermined minimum time period is achieved by means of a guardinterval inserted by a COFDM modulator between modulated symbols. 10.The apparatus of claim 1, wherein for the given arrangement of saidplurality of antenna units, each antenna unit communicates saidsuccessive modulated data blocks of a mobile terminal within range ofany of the plurality of antenna units over the single frequency band ofthe given data channel.
 11. A method of communicating data between afirst, mobile, terminal unit and a second terminal unit over a datachannel established by means of a plurality of antenna units, havingdifferent areas of coverage, and an associated base station to thesecond terminal unit, the method comprising the steps of: (i) at thefirst, mobile, terminal unit, generating a modulated data signalaccording to a predetermined modulation scheme; (ii) transmitting themodulated data signal wirelessly for reception by at least one of saidplurality of antenna units using a single frequency band of said datachannel; and (iii) at the associated base station, demodulating thereceived modulated data signal in said data channel for communication tothe second terminal unit, wherein, at step (i), according to saidpredetermined modulation scheme, successive blocks of modulated data insaid data channel are arranged such that a predetermined minimum timeperiod elapses between the arrival, at the base station, of a first andthe arrival of a second of said successive modulated data blocks andwherein, for a given arrangement of said plurality of antenna units,said predetermined minimum time period is set to a time interval at thebase station from the time of first arrival to the time of latestarrival of the first of said successive modulated data blocks in saiddata channel by means of antenna units in said given arrangement suchthat all data in a given data channel is communicated via at least oneof the plurality of antenna units at one time.
 12. A method according toclaim 11, wherein the time of latest arrival of the first of saidsuccessive modulated data blocks in said data channel is the time oflatest arrival of the first of said successive modulated data blocks ata signal level sufficiently high to cause interference with the secondof said successive modulated data blocks arriving in said data channel.13. A method according to claim 11, wherein a data signal may becommunicated over a selected one of at least two data channels andwherein, at step (i), the modulated data signal is converted to a signalof a respective predetermined frequency allocated for communicating dataover the selected data channel.
 14. A method according to claim 13,wherein said predetermined frequencies lie in the frequency range 55 GHzto 65 GHz.
 15. A method according to claim 13, further comprising thesteps of: (iv) receiving, at the base station, the modulated data signalcommunicated over the selected data channel; (v) routing the receivedmodulated data signal through a channel separator arranged to separatereceived modulated data signals according to their channel frequency;(vi) in the event that the received modulated data signal is receivedfrom two or more of said plurality of antenna units, combining themodulated data signals received for the selected data channel; and (vii)demodulating the received modulated data signal, in particular whencombined, for the selected data channel.
 16. A method according to claim11, wherein said predetermined modulation scheme is a coded orthogonalfrequency division multiplexing (COFDM) scheme and wherein saidpredetermined minimum time period is achieved by means of a guardinterval inserted at step (i) between modulated symbols when generatingthe modulated data signal.
 17. The method of claim 11, whereintransmitting the modulated data signal comprises: communicating saidsuccessive modulated data blocks of the first mobile terminal unitwithin a range of any of the plural antennas over the single frequencyband of said data channel.